This invention relates to a synchronizing signal detecting apparatus.
A description will now be made of a frame synchronizing signal detecting apparatus of a DAB signal (abbreviation of "Digital Audio Broadcasting" signal developed in the EUREKA 147 project).
FIG. 10 is a block diagram of the conventional synchronizing signal detecting apparatus. Reference numeral 1 is an input terminal of a DAB signal, reference numeral 2 shows an envelope detecting unit connected to the input terminal 1, reference numeral 3 denotes a comparator connected to the envelope detecting unit 2, reference numeral 4 represents a switch for switching an output signal between an S1 side of the comparator 3 and an S2 side of the ground, and reference numeral 5 shows an edge detecting unit connected to an output terminal of the switch. Reference numeral 6 is a timer, reference numeral 7 shows a control unit connected to a control terminal of the timer 6, into which the detection signal of the edge detecting unit 5, and reference numeral 8 indicates an output terminal of a frame synchronizing signal.
The received DAB signal is input into the input terminal 1. Since a NULL period during which a level of the DAB signal is 0 is provided at a head of the DAB signal frame, an output signal of the comparator 3 which has passed the envelope detecting unit 2 has a waveform (a) as shown in FIG. 11. Since a falling edge having an L level of this comparator output signal corresponds to a head of the NULL period, namely a head of a frame of the DAB signal, this falling edge is detected, so that synchronizing timing of the frame can be detected.
However, depending upon the reception conditions and the noise, the L level may be mixed in the envelope other than the NULL period and the L level of the NULL period may be dropped out. Therefore, the DAB signal is processed by the conventional synchronizing signal detecting apparatus as follows:
First, as indicated by waveform (b) in FIG. 11, the process operation is explained as executed when the L level is mixed into the comparator output signal.
Also assume that, as represented by waveform (c) of FIG. 11, the switch 4 is connected to the output S1 side of the comparator 3, and the falling edge of the comparator output signal into which the L level is first mixed is detected at a time instant "t0". Upon receipt of this detection signal, the control unit 7 resets the timer 6, and at the same time, causes the switch 4 to be connected to the ground S2 side after .DELTA.T/2. Thereafter, every time the frame period T from t0 is counted, the control unit 7 causes the switch 4 to be connected to the S1 side during only the time period of .DELTA.T, and at the same time, resets the timer 6.
Next, since the switch 4 has been switched to the S2 side even when the time instant becomes "t1", even if the comparator output signal is at the L level, the edge detecting unit 5 cannot detect the falling edge of the comparator output signal.
Next, when the time instant becomes "t0+T", although the switch 4 is connected to the S1 side only during the time period .DELTA.T, since the comparator output signal is in the H level, the edge detecting unit 5 cannot detect the falling edge of the comparator output signal.
Then, when the time instant becomes "t0+2T", although the switch 4 is connected to the S1 side only during the time period .DELTA.T, since the comparator output signal is in the H level, the edge detecting unit 5 cannot detect the falling edge of the comparator output signal.
In such a case as described above, the control unit 7 could not continuously detect the falling edge two times, and thus the control unit 7 judges that the time instant t0 is not equal to the head of the frame of the DAB signal, and thus causes the switch 4 to be connected to the S1 side.
Next, when the edge detecting unit 4 detects the falling edge at a time instant "t3", the control unit 7 resets the timer 6, and further causes the switch 4 to be connected to the S2 side after .DELTA.T/2.
Then, when the time instant of the timer 6 becomes "T", the control unit 7 causes the switch 4 to be connected to the S1 side, and at the same time, resets the timer 6. Therefore, when the time instant becomes a time instant "t4", the edge detecting unit 5 detects the falling edge. At this time, since the control unit 7 continuously detects the falling edge at the time instants t3 and t4, the control unit 7 judges that the time instant t1 corresponds to a head of a frame of the DAB signal. Thereafter, when the output from the timer 6 becomes T, after the timer 6 is reset, the control unit 7 outputs a frame synchronizing signal having an L level to the output terminal 8 only during a predetermined time period.
As explained before, the conventional synchronizing signal detecting apparatus can detect the synchronizing signal of the correct frame even if the L level is mixed into the comparator output signal of the synchronizing signal. However, when there are many mixed L levels, the conventional synchronizing signal detecting apparatus can hardly detect the head of the frame, and detection takes much time.
Next, a description will now be made of the process operation in such a case that an L level of a comparator output signal is dropped out, as shown by waveform (d) in FIG. 11.
It is now assumed that, as represented by waveform (e) of FIG. 11, the switch 4 is connected to the S1 side before .DELTA.T/2 of the time instant t1, and the falling edge is detected at the time instant t1, the control unit 7 resets the timer 6, and causes the switch 4 to be connected to the S2 side after .DELTA.T. Subsequently, every time the timer 6 counts the frame period T, the control unit 9 resets the timer 6, and repeats such an operation that the switch 4 is connected to the S1 side during the time period of .DELTA.T.
Thereafter, when the time instant becomes t2-.DELTA.T/2, the switch 4 is connected to the S1 side. However, even when the time instant becomes t2, since the falling edge is dropped, the edge detecting unit 5 cannot detect the falling edge. At this time, the control unit 7 judges that the time instant t1 is not equal to a head of a frame of the DAB signal, and this resets the timer 6.
Next, when the time instant becomes t3-.DELTA.T/2, the switch 4 is connected to the S1 side. When the time instant becomes t3, the edge detecting unit 5 detects the falling edge, and the control unit 7 resets the timer 6 and causes the switch 4 to be connected to the S2 side during the time period of .DELTA.T. When the time instant of the timer 6 becomes t4-.DELTA.T/2, the control unit 4 resets the timer 6 and causes the switch 4 to be connected to the S1 side. When the time instant becomes t4, if the edge detecting unit 5 detects the falling edge, then the control unit 7 continuously detect the falling edges two times at the time instants t3 and t4. As a result, the control unit 7 judges that the time instant t3 corresponds to a head of a frame of the DAB signal. Subsequently, every time the output of the timer 6 becomes T, the control unit 7 resets the timer 6, and thereafter outputs a frame synchronizing signal having an L level to the output terminal 8 only during a predetermined time period.
As explained before, the conventional synchronizing signal detecting apparatus can detect the synchronizing signal of the correct frame even if the L level is mixed into the comparator output signal of the synchronizing signal. However, when there are many mixed L levels, the conventional synchronizing signal detecting apparatus can hardly detect the head of the frame, and such detection takes much time.
As previously described, the conventional synchronizing signal detecting apparatus has such a problem that when the noise is mixed into the synchronizing signal and the synchronizing signal is dropped out due to the reception conditions and the noise, a lengthy time is required so as to detect the synchronizing signal.